Heretofore, a synthetic polyisoprene rubber (IR) was developed as a synthetic rubber having a structure similar to a natural rubber and widely used in rubber articles such as a tire and the like. The dynamic properties of the polyisoprene rubber are mainly dependent on a cis-1,4-bond content and a molecular weight, and the processability thereof is dependent on a branch ratio, gel quantity and so on in addition to the molecular weight.
Although the synthetic polyisoprene rubber has the structure similar to the natural rubber as mentioned above, the cis-1,4-bond content of the synthetic polyisoprene rubber is about 98% at most, while the cis-1,4-bond content of the natural rubber is not less than 99%, and it is thought that the dynamic properties of the polyisoprene rubber are inferior to those of the natural rubber due to the slight difference in the structure. Therefore, it is further demanded to develop polyisoprene rubbers having a higher cis-1,4-bond content and excellent dynamic properties. Also, polyisoprene rubbers being less in the branch ratio and gel quantity and excellent in the processability are demanded from a viewpoint of industrial practicality.
Although it is known that a rubber composition having an excellent balance between the processability and the dynamic properties is obtained by blending the polyisoprene rubber with natural rubber, the rubber composition has a problem that the dynamic properties of the rubber composition are deteriorated as the ratio of the polyisoprene rubber in the rubber composition is increased. As a cause on such a deterioration are considered a difference in the micro-structure such as a 3,4-bond content or the like in addition to the cis-1,4-bond content, a difference of other structural factor derived from the natural rubber and so on, but it is unclear which is main reason.
On the other hand, there is developed a technique of producing a polyisoprene rubber having a cis-1,4-bond content of not less than 98.5% (see WO02/38635A). Furthermore, a polyisoprene rubber having a very high cis-1,4-bond content, which has about 100% of cis-1,4-bond content is recently developed and announced (see Shojiro Kaita, Yoshiharu Doi, Kumiko Kaneko, Akira Horiuchi and Yasuo Wakatsuki, lecture abstract of 83rd spring annual meeting of Chemical Society of Japan; Institute of Physical and Chemical Research, “A synthetic rubber for a future high-performance tire has been accomplished”, [online], Mar. 18, 2003, Internet <URL:http://www.riken.go.jp/w-world/info/release/press/2003/030318/index.html>). However, the balance between the dynamic properties and the processability in the rubber composition comprising a blend of natural rubber and the polyisoprene rubber was not utterly mentioned in the official gazette and announcement, so that its industrial value is unclear.